BPF selftests and strict aliasing

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

Hello.
The following BPF selftests perform type-punning:

  progs/bind4_prog.c
  136 |         user_ip4 |= ((volatile __u16 *)&ctx->user_ip4)[0] << 0;

  progs/bind6_prog.c
  149 |                 user_ip6 |= ((volatile __u16 *)&ctx->user_ip6[i])[0] << 0;

  progs/dynptr_fail.c
  549 |         val = *(int *)&ptr;

  progs/linked_list_fail.c
  318 |         return *(int *)&f->head;
  329 |         *(int *)&f->head = 0;
  338 |         f = bpf_obj_new(typeof(*f));
  341 |         return *(int *)&f->node2;
  349 |         f = bpf_obj_new(typeof(*f));
  352 |         *(int *)&f->node2 = 0;

  progs/map_kptr_fail.c
   34 |         *(u32 *)&v->unref_ptr = 0;

  progs/syscall.c
  172 |         attr->map_id = ((struct bpf_map *)&outer_array_map)->id;

  progs/test_pkt_md_access.c
   13 |                 TYPE tmp = *(volatile TYPE *)&skb->FIELD;               \

  progs/test_sk_lookup.c
   31 |         (((__u16 *)&(value))[LSE_INDEX((index), sizeof(value) / 2)])
  427 |         val_u32 = *(__u32 *)&ctx->remote_port;

  progs/timer_crash.c
   38 |         *(void **)&value = (void *)0xdeadcaf3;

This results in GCC warnings with -Wall but violating strict aliasing
may also result in the compiler incorrectly optimizing something.

There are some alternatives to deal with this:

a) To rewrite the tests to conform to strict aliasing rules.

b) To build these tests using -fno-strict-aliasing to make sure the
   compiler will not rely on strict aliasing while optimizing.

c) To add pragmas to these test files to avoid the warning:
   _Pragma("GCC diagnostic ignored \"-Wstrict-aliasing\"")

I think b) is probably the best way to go, because it will avoid the
warnings, will void potential problems with optimizations triggered by
strict aliasing, and will not require to rewrite the tests.

Provided [1] gets applied, I can prepare a patch that adds the following
to selftests/bpf/Makefile:

  progs/bin4_prog.c-CFLAGS := -fno-strict-aliasing
  progs/bind6_prog.c-CFLAGS := -fno-strict-aliasing
  progs/dynptr_fail.cw-CFLAGS := -fno-strict-aliasing
  progs/linked_list_fail.c-CFLAGS := -fno-strict-aliasing
  progs/map_kptr_fail.c-CFLAGS := -fno-strict-aliasing
  progs/syscall.c-CFLAGS := -fno-strict-aliasing
  progs/test_pkt_md_access.c-CFLAGS := -fno-strict-aliasing
  progs/test_sk_lookup.c-CFLAGS := -fno-strict-aliasing
  progs/timer_crash.c-CFLAGS := -fno-strict-aliasing

[1] https://lore.kernel.org/bpf/20240127100702.21549-1-jose.marchesi@oracle.com/T/#u